Continuously operating agitator ball mills

ABSTRACT

An agitator ball mill including an annular stator assembly and a co-axially mounted rotor is disclosed. A first annular member is mounted on the stator and a second annular member is mounted on the rotor shaft. The position of the second annular member along the shaft is determined by a demountable spacer member to provide coarse adjustment of the width of a gap defined between mutually facing axial surfaces of the first and second annular members. Coolant is applied to the stator to cause controllable axial contraction of the stator relative to the shaft. This relative contraction increases the width of the gap and thus provides a fine adjustment of the width, so as to prevent grinding bodies from passing through it, whilst permitting material ground in the mill by the bodies to pass through the gap and thereby become separated from the grinding bodies.

United States Patent Schieritz Oct. 2, I973 [75] Inventor: Martin K. Schieritz, Basel,

Switzerland [73] Assignee: Willy A. Bachofen, BaseLSwitzerland [22] Filed: May 18, I972 21 Appl. No.: 254,644

130] Foreign Application Priority Data Mar. 1, I972 Switzerland 2944/72 [52] U.S. Cl. 241/66, 241/36 [51] Int. Cl. B02c 7/14, B020 ll/O8 [58] .Field of Search .241/36, 66, I70, 241/189, DIG. 2, DIG. 7

[56] References Cited UNITED STATES PATENTS l,937,788 l2/l933 Ross .t 241/66 X 2,083,l7l 6/1937 Nester 241/66 2,403,9l4 7/l946 Eppenbach et al. 24l/66 X Primary Examiner-Othell M. Simpson Assistant Examiner-4E. F. Desmond Attorney-Michael S. Striker [57] ABSTRACT An agitator ball mill including an annular stator assembly and a co-axially mounted rotor is disclosed. A first annular member is mounted on the stator and a second annular member is mounted on the rotor shaft. The position of the second annular member along the shaft is determined by a demountable spacer member to provide coarse adjustment of the width of a gap defined between mutually facing axial surfaces of the first and second annular members. Coolant is applied to the stator to cause controllable axial contraction of the stator relative to the shaft. This relative contraction increases the width of the gap and thus provides a fine adjustment of the width, so as to prevent grinding bodies from passing through it, whilst permitting material ground in the mill by the bodies to pass through the gap and thereby become separated from the grinding bodms.

15 Claims, 1 Drawing Figure CONTINUOUSLY OPERATING AGITATOR BALL MILLS BACKGROUND OF THE INVENTION The present invention relates to an agitator ball mill for extremely fine grinding or the disintegration of extremely small particles.

Ball mills are known wherein in a grinding recepta' cle, grinding bodies, in most cases glass balls, are set in motion by way of an agitator shaft. The energy imparted to the balls enables them to comminute the material to be ground through the effect of impact and shearing force. It is self-evident, in this connection, that the size of the grinding bodies must be dependent upon the size of the particles in the starting product. The smaller the particle size in the grinding stock, the smaller, also, must be the diameter of the grinding bodres.

With decreasing diameters of the grinding bodies, however, increasing difficulty was encountered over the separation of the ground stock from the grinding bodies at the end of the grinding operation. When grinding bodies with a diameter of less than 0.2 mm are employed it is no longer possible to use a simple sieve because this would have become clogged by the balls immediately. It is known to cause the ground material to pass, for the purpose of separation, through a narrow gap having a width less than the diameter of the grinding bodies. By moving the two walls of the gap relative to one another it was possible to prevent the balls from stopping up the gap. In practice a screening gap of this type was produced by means of two co-axial discs a short distance apart, one of them having a concentric orifice for the passage of the emerging ground material. One of the two discs was then set in rotation for the purpose of screening in order to generate the relative movement of the walls of the gap. With such an arrangement it was possible to guarantee continuous operation of ball mills, even for the finest grinding stock.

To ensure that all the grinding bodies remain in the grinding receptacle on separation an empirical value for the width of the gap of approximately one third of the diameter of the grinding ball was found to be convenient. Now, for the disintegration of bacteria quite small grinding bodies are needed, that is to say balls with a diameter of 0.1 millimetre. On the basis of the above-stated facts, this requires a gap width of approximately 0.03 millimetre, which with the highly developed machines in use at present can be achieved easily with the greatest precision.

In the case of disintegration of micro-organism such as bacteria, yeasts, etc., for example, a further problem arises, however. To prevent the inactivation or destruction of the products, e.g., enzymes, released from the cells on disintegration, these must in no circumstances be heated above a certain maximum temperature. In most cases this temperature is t-lr5C. Owing to the heat energy generated during the grinding process this necessitates a cooling device for the grinding receptacle and also for the other moving and stationary parts of the mill which might transmit heat to the grinding stock.

With conventional mills with cooling, it has been found that, in spite of the observance of closest tolerance in the manufacture of the mechanical parts and in spite of the most careful screening of the grinding bodies, the system worked satisfactorily at first but after a time the grinding bodies emerged progressively from the mill along with the ground material. Exhaustive investigations showed that as a result of the cooling of the mill individual parts contracted in an uncontrolled manner, thus causing the gap to widen, so that with increasing cooling more and more grinding bodies were able to pass through the gap. This disadvantage is naturally unacceptable, particularly in the case of precision mills of this type.

An object of the present inventionis therefore to create a ball mill of the type mentioned initially, in which the adverse side effects produced by cooling are significantly reduced.

SUMMARY OF THE INVENTION According to the present invention, there is provided an agitator ball mill comprising a generally annular stator assembly including a first annular member, a rotor shaft mounted co-axially with respect to the stator assembly, a second annular member mounted on the rotor shaft to rotate therewith, mutually facing axial surfaces of the first and second annular members defining therebetwee'n a gap, a demountable spacer member axially locating the second annular member relative to the rotor shaft, and means for applying a cooling medium to the stator assembly to cause the latter to contract axially relative to the rotor shaft and thereby to increase the width of the gap, so that in use the width of the gap may be so adjusted as to prevent grinding bodies from passing through it whilst permitting material ground in the mill by the bodies to pass through the gap and thereby become separated from the grinding bodies.

In this connection it is advantageous for the rotor to be connected co-axially with an agitator shaft. Ballbearings, especially double ball-bearings, are suitable for the precise mounting of the agitator shaft. To reduce wear, the two surface portions between which the gap is defined can consist of hard metal or some other wear-resistant material.

BRIEF DESCRIPTION OF THE DRAWINGS An exemplified embodiment of the invention will now be more particularly described with reference to the single FIGURE of the accompanying drawings, which shows a section through the axis of a ball mill embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The individual parts of the mill are mounted ona frame I, only a portion of which is represented in the drawing. A bearing body 4 which is essentially a hollow cylinder, is fixed to the frame 1 by bolts 2 and a support ring 3. A part 5 of the bearing body 4 which narrows by steps from the outside extends through the wall of the frame 1 through a co-axial opening la in the frame.

A concentric sleeve 6 is pushed over the bearing body 4. The internal diameter of this sleeve is not constant, however, but at the free end of the 'sleeveit corresonds to the external diameter of the bearing body 4 at this point and likewise at the other end, where the sleeve 6 is also sunk in an annular groove lb in the frame. The free end faces of the bearing body 4 and the sleeve 6 lie in one plane. Formed in the bearing surfaces of the sleeve on the bearing body are two grooves 6a and 6b which accommodate in each case an O-ring 7 and 8 which seal the two parts off from one another. Between the two ends of the sleeve 6 its internal diameter is widened and the external diameter of the bearing body 4 is reduced so that between the two parts there is a generally annular cavity ill provided with inlet and outlet connections 9 and 10 respectively. These connections are used for conveying a cooling liquid into and out of the cavity ill.

in the bearing body 4, close to its end which is further from the frame, a double ball bearing 12 is fixed by its outer ball race 12a. To fix it in a precisely defined position, so as to be immovable in the axial direction, the inner wall of the bearing body 4 has two graduations, their end faces acting as supports for two rings 13 and 14 which are in contact with the two end faces of the ball-bearing 12, thereby holding the latter in position. The ring 13 is fixed to the bearing body 4 by means of screws 13a. The inner ball race 12b of the ball-bearing 12 is a snug fit on a hollow shaft 15 which extends right through the bearing body and projects beyond it at both ends. At the level of the ring 113 the hollow shaft 15 has a torus 115a which acts as a stop for the inner ball race 12b. On its other side the latter is secured against axial displacement by means of a spacer sleeve 16 mounted co-axially on the hollow shaft 15. The inner space of the double ball-bearing is protected by means of two cover rings ll'7a and 17b from the entrance of foreign bodies.

At the other end of the bearing body, that is to say in its sector 5, is fitted a further ball-bearing 18a, 18b. The outer ball race 18a is fitted into the somewhat widened inner space of the part 5, whereas the inner ball race rests on the hollow shaft 115 and is braced on the end face of the spacer sleeve 16 on the side facing the double ball-bearing. Fixed on the end of the hollow shaft 15 projecting from the ball-bearing 18 is a fourstep pulley 19 which is connected with the hollow shaft so as to be rigid in respect of rotation by means of a slot-and-key connection 39b. The end face of the hub 19a of the pulley, which faces the frame l, is supported on the inner ball race 18b of the ball-bearing 18a, Mb. The pulley 19 is driven by a motor 652 which is represented'only diagrammatically on the drawing.

Fitted on the common end face of the two parts 4 and 6 is an annular fixing disc 20 which is attached to the bearing body by screws 211. For this purpose threaded bushes 2111 are let into the bearing body to ensure greater stability and precision of the joint. To make it possible for the fixing disc to be fixed in the correct position the sleeve 6 is provided with a centering pin 22 which engages in a corresponding hole in the disc 20, thus defining its position clearly. The disc 20 has a bigger diameter than the sleeve 6. The annular part of the disc projecting beyond the latter has drilled holes 23 to accommodate bolts 23a. By means of these bolts a flange 24, of substantially hollow cylindrical shape, is secured to the fixing disc 20, its internal diameter in its portion facing the double ball-bearing corresponding approximately to the mid-line diameter of the latter and in its other part approximately to the external diameter of the ball-bearing 12. This flange, together with the bearing body 4, the sleeve 6 and the fixing disc 20, constitutes the stator.

At the point of transition between the two diameters is a graduated constriction 24a on which is supported by its broad sides an annular lip seal 25a. Adjacent one end face of the lap 25a is a ball spacer 26 which makes contact by its outer wall with the inner wall of the flange 24. If required, a second lip seal 25b adjoins the ball spacer 26, or as a substitute for this a blank flange with an elastic ring 27a adjoining it. A thrust collar 27 rests against the ring 27a by the inner portion of one graduated end face and against the end face of the flange 24 facing the fixing disc 20 by the outer portion and is connected with the flange by means of screws 28.

The hollow shaft 15 extends as far as the constriction 24a of the flange 24. in the region beyond the torus 15a, a shaft protecting sleeve 29 is fitted tightly on the shaft. The sleeve 29 consists of polished hard metal and projects slightly over the end of the hollow shaft 15. The lips of the two sealing rings 25a and 25b rest against the smooth surface of the sleeve 29. Between one end face of the shaft protecting sleeve and the torus 15a a disc 30 is mounted on the hollow shaft 15 in such a way that its end face is supported on the shaft protecting sleeve 29 on one side and on the torus on the other. The function of this disc 30 is to throw any grinding stock which may pass through the lip seal radially outwards, where it can run off on the inner wall of the bearing body 4 and can drain away into the outer space of the mill through an opening 31 formed in the body 4 and in the sleeve 6. This provides simultaneously the possibility of controlling the state of the lip seals 25a and 25b. A further function of the disc 30 consists in the fact that with it it is possible to compensate for all the longitudinal dimensions of those machine parts which cannot be produced with the precision required for adjustment of the gap.

In the flange 24 are two radial bores 32a and 32b which connect with the interior of the flange situated between the two lip seals. These bores are provided with connections 33a and 33b which make it possible to lubricate continuously the seals, especially the seal 25b. In cases where special cooling of all parts of the mill is important, these bores 32a and 32b can also be utilized for the intake and outlet respectively of a cooling medium which cools the area of the shaft protecting sleeve 29, or the hollow shaft 15, between the two seals where heat is generated through the friction.

Opening into the inwardly widened space'35 of the flange 24 from above is a bore 34a which passes, towards the outside, into a connecting nipple 34 for the removal of the ground material.

Close to the axis the free end face of the flange 2 4 has a bush-type projection 24b into which there is let a groove 24c in which, in turn, an O-ring 36 is inserted. On this projection 24b and with one end face in contact with the flange 24 a bearing ring 37, which is sealed off by the O-ring 36, is fitted, with an annular groove 38 in the mid-line of its free end face. Inserted in the groove 38 is a further O-ring 39. The bearing ring 37 acts as a mating pressure face for the grinding receptacle 40 which is substantially cylindrical and open at one end face, its opening edge resting tightly on the O-ring 39 and on the bearing ring 37. The support for the receptacle is constituted by a clamping device which is not shown in the drawing. It acts on the closed end of the receptacle and presses it against the bearing ring. The receptacle is double-walled and is provided with an inlet and an outlet for a cooling liquid which'ca n be introduced between its two walls, these also not being shown in the drawing.

Mounted inside the hollow shaft 15 is the actual agitator shaft 41, with quite a small amount of play. At one end it projects somewhat beyond the hollow shaft and the pulley 19 and is likewise connected in a rotationally fixed manner by way of the slot-and-key connection 1% with the hollow shaft, or the pulley l9. Screwed on to the projecting portion is a tension nut 42 which is supported on the end face of the hub of the pulley.

The other end of the agitator shaft 41 projects from the flange 24 to just before the bottom of the grinding vessel 40 and there has a disc-like widened end 411a.

Adjacent to the shaft protecting sleeve 29 on the agitator shaft there is mounted a distance piece 43 which widens out conically in the direction of the end of the agitator shaft. Formed in its end face which is in contact with the shaft protecting sleeve is a recess of the diameter ofthe hollow shaft, in which is inserted a sealing sleeve 44 which lies with its other end between the shaft protecting sleeve and the agitator shaft and is supported on the free end of the hollow shaft 15. Consequently the distance piece 43 is tightly connected with the shaft protecting sleeve 29.

Adjacent to the bearing ring 37, on a corresponding groove in the projection 24b of the flange 24, is a hard metal first annular member 45 with an approximately square cross-section. Thefree end face of the ring 45 is the foremost front surface on the flange 24 and it constitutes the fixed wall of the gap 46.

The second wall of the gap is formed by a further hard metal ring 47 which is fixed to a bracket 48 which, in turn, is precisely situated on the agitator shaft. The end face of the ring 47 constituting the second wall of the gap 46 has a smaller internal diameter than the external diameter of the distance piece, so that the latter supports the ring 47by its free end face, presses it against the bracket 48 and thus holds it firmly and, through the longitudinal dimension of the distance piece 43, keeps it at a precisely defined distance from the ring 45. The ring 47, together with the remaining rotatable parts, constitutes the rotor assembly.

Adjacent to the bracket 48, small spacer tubes 49 only one of which is shown in the drawing are pushed on to the agitator shaft 61, between which are fixed agitator discs 50, of which, again, only one is illustrated, namely the last one, which rests against the blank disc 41a of the agitator shaft 41.

By the tightening of the tension nut 42 all the parts mounted on the agitator shaft, in so far as they are not already connected by means of drivers, are clamped to one another in a rotationally fixed manner and are thus able to participate in the rotation of the pulley 19.

The already ground material which is in the grinding receptacle 40 is now able to pass through the gap 46, the grinding bodies being held back, and it passes into the collecting space 35. When this is filled the material can flow out through the bore 34a into the connecting nipple 34.

If the tension nut 42 is removed the entire agitator shaft 41, with all the parts mounted thereon, can be withdrawn from the hollow shaft 15 after the grinding receptacle 40 has been taken off. The individual parts, such as for example the'sealing sleeve 44, the distance piece 43, the bracket 48 with the ring 47, the small tubes 49 and the agitator disc 50, can be pulled off the agitator shaft and, if necessary or desired, exchanged for new parts orparts of some other materials. This offers advantages from the point of cleaning techniques, permits a quicker exchange of worn parts, and facilitates the adaptation to special conditions which may be required.

This also applies to the shaft protecting sleeve 29 which is subject to unavoidable wear due to friction with the sealing lips 25a and 25b and the grinding stock, as well as the corresponding distance piece 43 which determines the gap width through the constant length of the shaft protecting sleeve 29 and the disc 30. The sleeve 29 and the disc 30 may be ground to predetermine their prescribed axial dimensions.

By replacing the agitator shaft. 41 by a longer or shorter one, with correspondingly more or fewer agita tor discs, or spacer tubes, and grinding receptacles of different sizes, the mill can be adapted excellently to any existing conditions.

Obviously it is also possible to mount the screening gap with the rotor and stator in some other way, without its having to run co-axially with the agitator shaft.

Now if, for example, it is required to disintegrate bacteria, the necessary size of grinding body for the purpose, e.g. 0.1 millimetre is first determined. Then, using the rule of thumb mentioned initially, a gap width of approximately 0.02 0.03 millimetre is obtained. When the specific grinding vessel and associated agitator shaft have been decided upon a distance piece 43 is selected, by means of which a gap width is defined with a slightly smaller value. This value depends, in fact, upon the type of cooling to be employed. If, for example, cooling is to be by tap water a distance piece must be inserted which produces a gap width of 0.02 millimetre at room temperature. If, however, cooling is effected with a special cooling brine at a temperature of20C, a distance piece must be used which reduces the gap width to zero at room temperature.

The agitator shaft is then inserted in the hollow shaft and tightened up by means of the tension nut. The cool ing liquid is then introduced into the space 11 between the bearing body and the sleeve 6 and for a definite span of time this part only is cooled and not simultaneously the grinding vessel, into which meanwhile the grinding bodies can be loaded and the suspension with the bacteria introduced by means of a pump.

The cooling effect spreads from. the bearing body 4 and the sleeve 6 by way of the disc 20 to theflange 24, but to a much lesser degree to the hollow shaft 15 and the agitator shaft 41. Consequently the flange 24 contracts relative to the agitator shaft and as a result the ring 45 is displaced axially by a small amount towards the pulley and the gap 46 thus widens. Here, the contraction of the bearing body 4 plays no part as it results in onlya slight displacement of its part 5, but the ballbearing 18 remains stationary relative to the ballbearing 12. 7

With tap water cooling, the increase in the width of the gap 46 amounts to approximately 0.005 millimetres and with cooling brine to about 0.03 millimetres, which in conjunction with the distance piece being used at any given time gives a gap width of 0.025 or 0.03 millimetres as the case may be.

Only now is the agitator shaft set in motion, or a start made with the cooling of the grinding vessel,since it is only now that thenecessary gap width has been established which will prevent any possible jamming of the two walls of the gap. Since in most cases, with microorganisms, there is an upper temperature limit of +5 C which may not be exceeded and, on the other hand, the flange 24 has to be cooled more than the agitator shaft,

it is only a cooling brine of 20C which can be considered in this case for cooling the stator. Obviously it is also possible to use relatively warmer cooling media for cooling the stator. or the flange 24, provided care is taken to see that the stator has a higher thermal coefficicnt ofexpansion than the distance pieces defining the position of the ring 47.

Then, by means of a pump not shown the suspension is pumped into the vessel through a filler neck 40a and there disintegrated (broken up) in a continuous process. Then, when the grinding bodies have been separated off, it leaves the grinding vessel through the nipple 34.

The advantages of the mill described above reside in the fact that the width of the gap can be adapted to exisiting circumstances in the simplest way by means of inter-changeable distance pieces. The fine adjustment of the gap width by means of the temperature of the cooling medium for the flange on which the fixed wall of the gap is fitted, offers the advantage that it can be carried out conveniently and above all in the most precise manner possible. By fitting an additional safety device, for example a temperature sensor 60 with a thermo-switch 61, it is possible to keep the temperature of the cooling medium and hence also the gap width under constant supervision and in the event of an unforeseen rise in the temperature of the cooling medium the two wallsof the gap can be prevented from becoming jammed, since this danger can be detected promptly and eliminated either manually or automatically by way of the stopping of the driving motor 62.

I claim:

1. An improvement in an agitator ball mill, the improvement comprising in combination;

a generally annular stator assembly including a first annular member;

a rotor shaft mounted co-axially with respect to said stator assembly;

a second annular member mounted on said rotor shaft to rotate therewith;

a demountable spacer member'axially locating said second annular member relative to said stator assembly; and

a passage through said stator assembly for circulating a cooling medium to cause the stator assembly to contract axially relative to said rotor shaft and thereby increase the width of a gap defined between mutually facing axial surfaces of said first and second annular members; whereby the width of said gap may be adjusted to prevent grinding bodies from passing through said gap whilst permitting material ground in the mill by the bodies to pass thcrethrough and thereby become separated from the grinding bodies.

2. An agitator ball mill as defined in claim 1, wherein said first and second annular members each comprise wear-resistant material.

3. An agitator ball mill as defined in claim 1, wherein said grinding bodies comprise balls, each said ball having a diameter of less than 0.2 millimetres.

4. An agitator ball mill as defined in claim 1, wherein said rotor shaft comprises an inner shaft and a hollow outer shaft mounted co-axially on said inner shaft to rotate therewith.

5. An agitator ball mill as defined in claim 4, comprising at least one agitator member mounted on said inner shaft to rotate therewith.

6. An agitator ball mill as defined in claim 4, wherein said second annular member is mounted co-axially on said inner shaft to rotate therewith and to be restrained from axial displacement with respect thereto.

7. An agitator ball mill as defined in claim 4, wherein said outer shaft is rotatably mounted in said stator assembly by a pair of axially spaced bearing means.

8. An agitator ball mill as defined in claim 7, wherein each said bearing means comprises a ball bearing assembly provided with an inner ball race and an outer ball race.

9. An agitator ball mill as defined in claim 8, wherein said ball bearing assembly adjacent said second annular member comprises a double-ball bearing, of which said outer ball race is restrained from axial displacement relative to said stator assembly.

10. An agitator ball mill as defined in claim 1, wherein a generally annular collecting cavity for receiving ground material passed through. said gap is defined between an end portion of said stator assembly and said rotor shaft, said collecting cavity being closed by a pair of axially spaced lip sealing members mounted on said stator assembly and contacting a shaft protecting sleeve fitted tightly but removable on said rotor shaft.

11. An agitator ball mill as defined in claim 10, wherein a further annular cavity is defined between said pair of lip sealing members, and wherein there is provided means for circulating coolant through said further cavity for dissipating heat produced by friction between said lip sealing members and the surface of said shaft protecting sleeve.

12. An agitator ball mill as defined in claim 4, wherein said inner shaft is secured against axial displacement by a nut engaging a threaded end portion of said inner shaft.

13. An agitator ball mill as defined in claim 12, wherein removal of the nut from said end portion of said inner shaft permits the latter, together with components of said rotor assembly mounted thereon, to be withdrawn from said hollow outer shaft.

14. An agitator ball mill as defined in claim 1, wherein said passage through said stator assebmly eomprises a generally annular chamber co-axial with said rotor shaft, said chamber being provided with an inlet and an outlet for circulating the cooling medium through said chamber.

15. An agitator ball mill as defined in claim 1, comprising temperature sensing means responsive to the temperature of the cooling medium exceeding a predetermined value to automatically disable drive means for rotating said rotor shaft. 

1. An improvement in an agitator ball mill, the improvement comprising in combination; a generally annular stator assembly including a first annular member; a rotor shaft mounted co-axially with respect to said stator assembly; a second annular member mounted on said rotor shaft to rotate therewith; a demountable spacer member axially locating said second annular member relative to said stator assembly; and a passage through said stator assembly for circulating a cooling medium to cause the stator assembly to contract axially relative to said rotor shaft and thereby increase the width of a gap defined between mutually facing axial surfaces of said first and second annular members; whereby the width of said gap may be adjusted to prevent grinding bodies from passing through said gap whilst permitting material ground in the mill by the bodies to pass therethrough and thereby become separated from the grinding bodies.
 2. An agitator ball mill as defined in claim 1, wherein said first and second annular members each comprise wear-resistant material.
 3. An agitator ball mill as defined in claim 1, wherein said grinding bodies comprise balls, each said ball having a diameter of less than 0.2 millimetres.
 4. An agitator ball mill as defined in claim 1, wherein said rotor shaft comprises an inner shaft and a hollow outer shaft mounted co-axially on said inner shaft to rotate therewith.
 5. An agitator ball mill as defined in claim 4, comprising at least one agitator member mounted on said inner shaft to rotate therewith.
 6. An agitator ball mill as defined in claim 4, wherein said second annular member is mounted co-axially on said inner shaft to rotate therewith and to be restrained from axial displacement with respect thereto.
 7. An agitator ball mill as defined in claim 4, wherein said outer shaft is rotatably mounted in said stator assembly by a pair of axially spaced bearing means.
 8. An agitator ball mill as defined in claim 7, wherein each said bearing means comprises a ball bearing assembly provided with an inner ball race and an outer ball race.
 9. An agitator ball mill as defined in claim 8, wherein said ball bearing assembly adjacent said second annular member comprises a double-ball bearing, of which said outer ball race is restrained from axial displacement relative to said stator assembly.
 10. An agitator ball mill as defined in claim 1, wherein a generally annular collecting cavity for receiving ground material passed through said gap is defined between an end portion of said stator assembly and said rotor shaft, said collecting cavity being closed by a pair of axially spaced lip sealing members mounted on said stator assembly and contacting a shaft protecting sleeve fitted tightly but removable on said rotor shaft.
 11. An agitator ball mill as defined in claim 10, wherein a further annular cavity is defined between said pair of lip sealing members, and wherein there is provided means for circulating coolant through said further cavity for dissipating heat produced by friction between said lip sealing members and the surface of said shaft protecting sleeve.
 12. An agitator ball mill as defined in claim 4, wherein said inner shaft is secured against axial displacement by a nut engaging a threaded end portion of said inner shaft.
 13. An agitator ball mill as defined in claim 12, wherein removal of the nut from said end portion of said inner shaft permits the latter, together with components of said rotor assembly mounted thereon, to be withdrawn from said hollow outer shaft.
 14. An agitator ball mill as defined in claim 1, wherein said passage through said stator assebmly comprises a generally annular chamber co-axial with said rotor shaft, said chamber being provided with an inlet and an outlet for circulating the cooling medium through said chamber.
 15. AN agitator ball mill as defined in claim 1, comprising temperature sensing means responsive to the temperature of the cooling medium exceeding a predetermined value to automatically disable drive means for rotating said rotor shaft. 